The present invention relates to a particle size measuring apparatus for measuring the diameter of small particles or microparticles.
A particle size measuring apparatus is known which measures the particle size distribution of particles such as water droplets in a turbine or fuel droplets in a combustion apparatus. One of the particle size measuring methods is the forward scattering method.
The principle of the forward scattering method is as follows.
A parallel monochromatic light such as a laser beam is irradiated on a number of particles to be measured. The angular pattern of the scattered light intensity, which is named the scatted light intensity pattern, is measured. The scattered light intensity pattern I(.theta.) is inverted to the particle size distribution n.sub.r (D) by solving Eq. (1) EQU I(.theta.)=.intg.i(D,.theta.)n.sub.r (D)dD, (1)
where i (D,.theta.) is the light intensity scattered by a D diameter spherical particle and .theta. is scattering angle. Eq. (1) expresses that the scattered light intensity pattern is superpositions of the light intensity i (D,.theta.) scattered by a D diameter particle. The light intensity i(D,.theta.) scattered by a spherical particle can be calculated by the Mie theory. Therefore, Eq. (1) can be solved. The present inventor previously proposed an apparatus for measuring particle size distribution n.sub.r (D) of a number of particles to be measured as stated in U.S. patent application No. 541,023, filed on Oct. 12, 1983, entitled "A Particle Diameter Measuring Device" now U.S. Pat. No. 4,595,291, issued June 17, 1986. In this apparatus, a laser radiated from a laser unit is converted into a parallel laser beam with an adequate diameter through an optical system such as a collimator lens. The parallel beam is irradiated on a number of particles to be measured. Scattered light intensities I.sub.1 to I.sub.n at respective scattering angles .theta..sub.1 to .theta..sub.n are measured by photodetectors arranged at the respective scattering angles .theta..sub.1 to .theta..sub.n and at equal distances from the center of a measurement area. Measured light intensity I(.theta.) (I.sub.1, I.sub.2, . . . , I.sub.n) is inverted to particle size distribution by the Logarithmic Constrained Inversion Method or the Logarithmic Distribution Function Approximation Method using Eq. ( 1).
In this apparatus, the laser beam from the laser unit is scattered by a collimator lens. The light scattered by the collimator lens causes background noise when the scattered light intensity pattern for a number of particles is measured. When a number of particles to be measured is small, this background noise becomes significant problem and the S/N ratio of the measured light intensity pattern is small.
In the above-mentioned apparatus, the scattered light intensity pattern is obtained as a relative value, and the particle size distribution inverted is also a relative value and can only be obtained, for example, as a particle size distribution density function.